Inrush currents or input surge currents are maximum instantaneous input currents drawn by an electrical device during power-up or when switching to another power source. Inrush currents may be several times the normal full load current and accordingly an overcurrent protection of the respective electrical or electronic device has to be provided. The overcurrent protection has to react quickly to the overload.
FIG. 1 is a simplified circuit diagram of a power supply selector 2 having a plurality of input nodes having input voltages VIN1, VIN2 and VIN3. In principal, there may be an arbitrary number of input nodes VIN1 to VINX. The power supply selector 2 is provided with an inrush current protection by applying a window comparator 4 comprising a first comparator CMP1 and a second comparator CMP2. Each of the power input nodes VIN1 to VIN3 of the power supply selector 2 is coupled to a first switch 6 having a first on-resistance and a second switch 8 having a second on-resistance. The first and the second switch 6, 8 are coupled in parallel. The on-resistance of the first switch 6 is lower than the on-resistance of the second switch 8. The first and second switch 6, 8 are further coupled to an output node having an output voltage VOUT1 for providing a system power source. Depending on whether one of the switches (i.e. the first switch 6 or the second switch 8) that is coupled to a respective one of the power input nodes VIN1 to VIN3 is opened or closed, the power output node VOUT1 is connected to one of the respective input sources VIN1 to VIN3.
The respective input source is selected by setting a power source select node VSEL1 from low to high. Accordingly, the second switch 8 having the higher on-resistance is closed. Preferably, the first and second switch 6, 8 is provided by suitable transistors, e.g. MOSFETs, having a different gate width. Since the on-resistance of the second switch 8 is comparably high, the inrush current is minimized due to its high resistance. However, the high on-resistance of the second switch 8 causes high losses and therefore it is not suitable for continuous operation. It is therefore desired to switch, i.e. to close, the first switch 6 having a low on-resistance in order to minimize these losses. However, if a peak current or inrush current is detected, the first switch 6 has to be opened in order to minimize the inrush current.
Inrush current protection is typically provided by sensing a voltage difference between the respective input node, i.e. by sensing one of VIN1 to VIN3 and the voltage VOUT1 at the power output node. According to the prior art, this is achieved by applying a window comparator 4. A selected one of the power input nodes VIN1 to VIN3 is coupled to the inverting terminal of the first comparator CMP1 and to the non-inverting terminal of the second comparator CMP2. This sense voltage VSENSE is compared to the voltage VOUT1 at the output node by coupling the output node to the non-inverting terminal of the first comparator CMP1 and to the inverting terminal of the second comparator CMP2. The output terminal of the first and second comparator CMP1, CMP2 as well as the inverted power source select signal VSEL1 (inverted by inverter 10) are coupled to the input nodes of a NOR-gate 12. According to the commonly known truth table of the NOR-gate, the output terminal thereof provides a switching signal SW opening the first switch 6 if either the output of the first comparator CMP1 or the second comparator CMP2 is set to high. This means that the difference between the sense voltage VSENSE and the output voltage VOUT1 is greater than a predetermined threshold value. According to the example in FIG. 1, this threshold value is near or equal to zero and the first switch 6 is opened, if the two voltages VSENSE and VOUT1 are not equal to each other. Obviously, the first switch 6 is also opened if the power supply select signal VSEL1 is set to low and accordingly the inverted power source select signal is high.
However, inrush current protectors according to the prior art have a large footprint since two comparators have to be implemented.